Demyelination in the central nervous system, resulting from genetic mutations or autoimmune disease (e.g. multiple sclerosis, MS), causes hearing impairments such as sudden hearing loss, inability to localize sound and/or tinnitus. Such deficits are generally attributed to the decrements in conduction velocity and temporal fidelity that accompany axon demyelination. However, the direct impact of demyelination on synapse functional parameters, including presynaptic excitability, neurotransmitter release and synaptic plasticity, is largely unknown in the auditory nervous system. To what extent do deficits in synapse function associated with demyelination contribute to hearing impairments? The objective of this proposal is to investigate the role of myelin ensheathment on the functional and structural maturation of the calyx of Held synapse in the auditory pathway. Our published and preliminary studies show that the loss of condensed myelination increases timing errors and failures in synaptic transmission at the calyx synapses in the auditory brainstem. Changes at the level of individual synapses are associated with delayed and degraded signal transmission along the auditory pathway in an in vivo auditory brainstem response test in myelin-deficient rats. Based on the preliminary data, we hypothesize that axon myelination is required for the proper location of ion channels at hemi-nodes and presynaptic terminals, for precision of transmitter release involved in short-term plasticity and for development of a tight synaptic structure. To test this hypothesis, we will use the Long-Evans Shaker rat, which completely lacks central myelination, excluding confounding effects associated with inflammatory factors. We will perform patch-clamp recordings of presynaptic terminals, post-synaptic neurons and astrocytes, along with Na+ and Ca2+ imaging. In Aim 1, we will test the hypothesis that lack of myelination disrupts the expression pattern of Na+ and K+ channels and leads to impulse timing errors and failures at presynaptic terminals. In Aim 2, we will test the hypothesis that lack of myelination decreases synaptic efficacy by increasing presynaptic Ca2+ accumulation and causing asynchrony of synaptic signals. In Aim 3, we will test the hypothesis that lack of myelin spatiotemporally disrupts synapse-astrocyte communication and leads to excessive glutamate-mediated axonal and synaptic degeneration. These studies will have a significant impact on our understanding of damaged auditory processing following central demyelination, and contribute to improve treatment of hearing disorders in MS and auditory neuropathy. Furthermore, an improved understanding the consequences of central demyelination at the synaptic level will have broad relevance for the entire field of neurodegenerative diseases.